Application NotesTechnical Documentation & Guides

Overview

Pressure decay testing is the most widely used production leak testing method. The principle is simple: pressurize the part to a specified pressure, isolate it from the pressure source, and measure any pressure drop over a defined test time. A pressure drop exceeding the reject threshold indicates a leak. Despite its simplicity, achieving reliable and repeatable results requires understanding the factors that affect measurement accuracy.

Test Theory

The pressure decay rate is related to the leak rate by the Ideal Gas Law. For a fixed test volume at constant temperature:

• Leak rate (scc/min) = (ΔP × V) / (Patm × t) where ΔP is pressure change, V is volume, t is time
• Smaller test volumes provide higher sensitivity (less air to lose)
• Higher test pressures increase sensitivity proportionally
• Longer test times allow smaller leaks to be detected
• Typical sensitivity: 10⁻² to 10⁻⁴ scc/min depending on conditions

Test Cycle Phases

A complete pressure decay test consists of four phases: Fill (pressurize the part), Stabilize (allow pressure and temperature to equalize), Test (measure pressure change), and Exhaust (depressurize). The stabilize phase is critical—without adequate stabilization, thermal effects will mask or mimic leaks, causing false results.

Factors Affecting Accuracy

Temperature is the primary source of error in pressure decay testing. A 0.1°F temperature change in a 100cc test volume at 30 PSI causes a pressure change equivalent to a significant leak. Other factors include adiabatic heating during fill (compressed air heats up), surface moisture evaporation, and flexible part deformation under pressure.

Typical Applications

• Automotive component testing
• Medical device seal verification
• Consumer product waterproofing
• HVAC component testing

Related Products

LM-Series Instruments • Sentinel Series

Overview

Different leak test methods offer different sensitivity levels, cycle times, and suitability for various part types. Selecting the right method requires balancing detection sensitivity, cycle time, cost, and the physical characteristics of the part being tested. LeakMaster instruments support multiple test methods to address the full range of production leak testing requirements.

Method Comparison

The four primary production leak test methods, ranked by typical sensitivity:

• Mass Flow: 10⁻³ to 10⁻⁵ scc/min — measures actual flow to maintain pressure
• Differential Pressure: 10⁻³ to 10⁻⁵ scc/min — compares test part against reference
• Pressure Decay: 10⁻² to 10⁻⁴ scc/min — measures pressure drop in isolated volume
• Vacuum Decay: 10⁻² to 10⁻⁴ scc/min — same as pressure decay but under vacuum

When to Use Each Method

Use pressure decay for general-purpose testing with moderate sensitivity requirements and simple fixturing. Use mass flow when you need direct leak rate measurement in engineering units (scc/min) or when test volumes are large. Use differential pressure when environmental temperature fluctuations would affect pressure decay accuracy. Use vacuum decay for parts that cannot be pressurized internally.

Part Characteristics to Consider

The part itself influences method selection. Rigid parts (metal castings, plastic housings) work well with all methods. Flexible parts (bags, tubing) may deform under pressure, requiring low test pressures or vacuum methods. Porous materials may require sealant or special fixturing. Parts with large internal volumes need longer test times or mass flow methods for adequate sensitivity.

Typical Applications

• Production line test planning
• Quality engineering specifications
• Test method validation
• Leak test process improvement

Related Products

LM-Series • Sentinel Series • Custom Test Systems

Overview

Integrating leak test instruments into automated production lines requires coordinating the test instrument with PLCs, robotic part handling, and test fixturing. Proper integration ensures reliable test results while minimizing cycle time impact on the production line. This guide covers communication interfaces, fixture design, and optimization techniques.

PLC Communication

LeakMaster instruments support multiple PLC communication options:

• Discrete I/O: Start, Pass, Fail, Ready signals for simple integration
• Serial (RS-232/485): Full parameter access and data retrieval
• Ethernet/IP: Direct tag access for Allen-Bradley PLCs
• Modbus TCP: Universal Ethernet protocol for any PLC brand
• PROFINET: For Siemens and other PROFINET controllers

Test Fixture Design

The fixture seals the part and connects it to the test instrument. Critical design considerations include: seal material selection (compatible with test medium and part surface), seal force (enough to prevent leaks without deforming the part), and internal volume minimization (smaller volume = higher sensitivity and faster tests). Use quick-connect fittings and minimize tubing length between the instrument and fixture.

Cycle Time Optimization

Test cycle time is typically dominated by the fill and stabilize phases. Reduce fill time by using larger diameter tubing and fittings. Reduce stabilize time by controlling the fill rate to minimize adiabatic heating, using regulators close to the fixture, and maintaining consistent ambient temperature. Consider overlapping test operations with part handling for maximum throughput.

Typical Applications

• Automotive production lines
• Medical device manufacturing
• Consumer electronics assembly
• HVAC production testing

Related Products

LM-Series • Sentinel Series • Custom Test Fixtures